Bit-rate and format insensitive all-option clock extraction circuit

ABSTRACT

A method and circuit are presented for the all optical recovery of the clock signal from an arbitrary optical data signal. The method involves two stages. A first stage preprocesses the optical signal by converting a NRZ signal to a PRZ signal, or if the input optical signal is RZ, by merely amplifying it. In a preferred embodiment this stage is implemented via an integrated SOA in each arm of an asymmetric interferometric device. The output of the preprocessing stage is fed to a clock recovery stage, which consists of a symmetric interferometer that locks on to the inherent clock signal by using the second stage input signal to trigger two optical sources to self oscillate at the clock rate. In a preferred embodiment the second stage is implemented via SOAs integrated in the arms of an interferometer, with two DFB lasers as terminuses. The output of the interferometer is an optical clock signal at the clock rate of the original input.

TECHNICAL FIELD

[0001] This invention relates to optical communications, and inparticular to a method of optical domain clock signal recovery fromhigh-speed data, which is independent of the data format or the opticalsignal rate.

BACKGROUND OF THE INVENTION

[0002] Optical fiber networks are in widespread use due to their abilityto support high bandwidth connections. The bandwidth of optical fibersruns into gigabits and even terabits. Optical links can thus carryhundreds of thousands of communications channels multiplexed together.

[0003] One of the fundamental requirements of nodal network elements inoptical networks is the capability to extract the line rate clock fromthe incoming signal. Presently, this is achieved by converting theincoming optical signal into an electrical signal followed by clockextraction using an application specific electronic circuit. As opticalnetworks become increasingly transparent, there is a need to recover theline rate clock from the signal without resorting toOptical-to-Electrical, or O-E-O, conversion of the signal.

[0004] Future optical networking line systems will incorporate servicesignals at both 10 Gb/s, 40 Gb/s and much higher data rates, along withthe associated Forward Error Corrected (FEC) line rate at each nominalbit rate. The FEC rates associated with, for example, 10 Gb/s opticalsignal transport include the 64/63 coding for 10 Gb/s Ethernet, the15/14 encoding of SONET-OC192 FEC, and the strong-FEC rate of 12.25Gb/s. As these networks tend towards optical transparency, the nodaldevices in the optical network must work with any commercially desiredline rate, independent of format, whatever that is or may be. Thus, oneof the fundamental functions these devices must provide is thecapability to extract the clock from an arbitrary optical signal.Moreover, to maintain the high speeds of modern and future datanetworks, as well as increase efficiency, this clock recovery must bedone completely in the optical domain.

[0005] In future All Optical Networks (AON) the same network elementwill need to handle both 10 Gb/s and 40 Gb/s. Consequently, the clockrecovery in these network elements must be tunable over a wide range offrequencies.

[0006] Previous embodiments of clock recovery systems are experimentalin nature, and relegated to research laboratories. They do not includethe possibility of recovering the line rate clock from the variousubiquitous NRZ data formats. Additionally, any tuning of the clocksignal is done using a linear phase section.

[0007] What is therefore needed is an all optical clock recovery systemthat can operate upon any given optical signal, regardless of its formator bit rate. What is further required is a system that exploitsnon-linear optical elements to reshape the clock output for optimalretiming of the various data formats.

SUMMARY OF THE INVENTION

[0008] A method and circuit are disclosed for the recovery of the clocksignal from an arbitrary optical data signal. The method involves twostages. The first stage consists of a Semiconductor OpticalAmplifier—Asymmetric Mach-Zehnder Interferometer, or SOA-AMZI,preprocessor, which is responsible for transforming an incoming NRZ typesignal into a pseudo return to zero (“PRZ”) type signal, which has asignificant spectral component at the inherent clock rate.

[0009] This preprocessing stage is followed by a second stage clockrecovery circuit. In a preferred embodiment the second stage isimplemented via an SOA-MZI circuit (symmetric in structure, i.e., nophase delay introduction in one of the arms) terminated by twoDistributed Feedback (DFB) lasers that go into mutual oscillationstriggered by the dominant frequency of the first stage's output signal.The SOA-MZI is tuned to adjust the input phase of the oscillatory signalinto the DFBs. This provides the tuning and control of the oscillationfrequency of the output clock signal. The SOA gain currents can beadjusted to reshape the clock signal, which is the output of the secondstage.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 depicts a circuit implementing the method of the presentinvention;

[0011]FIG. 2 depicts just the second stage of the circuit of FIG. 1; and

[0012]FIG. 3 depicts an exemplary semiconductor optical amplifier deviceused according to the method of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The above described and other problems in the prior art aresolved in accordance with the method, apparatus, circuit and devices ofthe present invention, as will now be described.

[0014] Most, if not all, optical networks currently operating transmitsome or all data as NRZ signals. In the case of the NRZ signal format,the RF spectrum reveals no spectral component at the line rate. This isa simple consequence of the format. The RF spectrum of an ideal NRZsignal looks like the mathematical sinc function with the first zero atthe line rate.

[0015] On the other hand, the RF spectrum of an RZ signal reveals astrong spectral component at the line rate. Consequently, an incoming RZsignal can be operated upon directly to extract the clock signal.

[0016] The fundamental problem of all-optical clock recovery from anarbitrary incoming optical signal is thus the passing of an RZ signalwithout attenuation, and the generation of a RF spectral component atthe line rate for a NRZ signal. For an NRZ signal of unknown bit rateand format, an NRZ/PRZ converter is used to generate this latterspectral component by converting the incoming NRZ into a pseudo returnto zero, or PRZ signal.

[0017] Once the incoming signal has a significant spectral component atthe line rate, optical oscillations can be triggered to obtain a pureline rate optical clock signal.

[0018]FIG. 1 depicts a preferred embodiment of the two circuit stagesneeded for all optical clock recovery of an arbitrary NRZ signal. Forvarious design considerations, most data in optical data networks iscurrently sent in the NRZ format. The first stage 150, converts an inputsignal 100 to PRZ format, where PRZ denotes a “pseudo return to zero” orPRZ data format. The PRZ signal is generated from a standard NRZ formatinput signal 100 by generating an RZ like pulse each time the NRZ signaltransitions, whether from high to low or from low to high, i.e. PRZ hasa pulse at each rising edge and at each falling edge of the originalsignal.

[0019] As above, the key property of a PRZ signal is that its RFfrequency spectrum has a significant frequency component at the originalNRZ signal's clock rate. It is this very property that the method of theinvention exploits to recover the clock signal.

[0020] The actual conversion of an NRZ signal to the PRZ format is theresult of the operation of a PRZ generator 150 on an NRZ input. Arelated patent application, under common assignment with the presentone, describes in detail a method and circuit for implementing thepreprocessor of the first stage 150. That patent application is entitled“FORMAT INSENSITIVE AND BIT RATE INDEPENDENT OPTICAL PREPROCESSOR” byBharat Dave, et al., filed on May 4, 2001. That disclosure is herebyfully incorporated herein by this reference. The method and circuitdescribed therein will thus be summarily described here for purposes ofreference.

[0021] The PRZ generator forms the first stage 150 of the All OpticalClock Recovery (“AOCR”) scheme. This stage consists of a path-delayedAsymmetric Mach-Zehnder Interferometer (AMZI). The AMZI incorporatessemiconductor optical amplifiers (SOAs) in each of its arms 105 and 106,respectively, and a phase delay element 107 in one, but not both, of thetwo arms; hence the asymmetry. The AMZI is set for destructiveinterference of the signals in the two paths. Thus, the interference ofa high bit with its path delayed inverse, i.e. a low bit, generates anRZ-like bit at both the leading and falling edges of the original highbit. This latter signal, with a bit rate effectively double that of theoriginal NRZ bit rate, is the PRZ signal 110.

[0022] This effective doubling of the bit rate leads to the generationof a large component of the line rate frequency in the RF spectrum ofthe output signal 110 of the AMZI 150. Generally, unless the inputsignal is exceptionally aberrant, this line rate frequency will be thefar and away dominant frequency in the spectrum. Since the preprocessordoes not need to know the actual bit rate or format of the input data,it is data rate and format insensitive.

[0023] Thus the preprocessor has the ability to reshape the PRZ signalas well as adjust its duty cycle. The output 110 of the first stage 150becomes the input to the second stage 160. In a preferred embodiment,the second stage 160 comprises a symmetric MachZehnder Interferometer,where each arm contains a semiconductor optical amplifier 111 and 112,respectively.

[0024] The principle of clock recovery is based on inducing oscillationsbetween the two lasers DFB1 113 and DFB2 114. The oscillations aretriggered by the output of the first stage 110. As described above, thisoutput can be either RZ or PRZ. The current to DFB2 114 is tuned closeto its lasing threshold, with DFB1 113 energized so as to be in lasingmode. Thus the trigger pulse 110 induces lasing in DFB2 114. Thefeedback from DFB2 114 turns off the lasing in DFB1 113 resulting inDFB2 114 itself turning off. The reduced feedback from DFB2 114 nowreturns DFB1 113 to lasing. In this manner the two lasers mutuallystimulate one another in oscillation. Recalling that the dominantfrequency in the input signal 110 is the original signal's 100 clockrate, pulses from the input 110 are sufficient to lock the oscillationof the DFB lasers at that rate, and, in general, to hold for quite anumber of low bits (such as would appear where the original signal 100had a long run of high bits). Thus, the forced triggering by the PRZ/RZinput 110 locks the phase of the oscillations at the original signal's100 clock rate.

[0025] The interferometer improves the control of the phase input toDFB2 114.

[0026] The use of the SOA-MZI facilitates the tuning of the oscillationrate by adjusting the input signal phase into DFB2 114. As the phase ofthe MZI output is tuned, the gain recovery time of DFB2 114 is adjusted.This results in the oscillation rate being altered. In this manner theclock frequency can be tuned to the desired line rate. Using non-linearSOA elements also allows shaping of the output clock with a lesserenergy expenditure. Moreover, by adjusting the currents in each of thetwo SOAs in the second stage interferometer, the refractive index ofeach SOA's waveguide can be manipulated, thus altering the phase of thepulse entering DFB2 114. Thus, the oscillation rate of the circuit canbe altered, and the identical circuit can be tuned to the various bitrates available in the network, thus rendering the system bit rateindependent.

[0027] The use of the SOA-AMZI in the first stage 150 of the clockrecovery system allows the input power required by the device to bequite nominal, in the embodiment depicted approximately −10 dBm; thussignal pre-amplification concerns are diminished or avoided. The outputpower of the clock signal in this embodiment is on the order of 0 dBm.The laser wavelength of the all-optical clock signal is a function ofthe wavelength amplification spectrum of the second stage SOAs. Withsuitably designed SOAs, the standard carrier frequencies used in opticalnetworks all fall within the SOA amplification spectrum. This wavelengthcan be anywhere in the amplification window of the SOAs in the secondstage 160 SOA-MZI circuit. Thus, as examples, for the C-band of opticaltransmission a wavelength such as 1550 nm may be chosen, and for theL-band of optical transmission a wavelength such as 1585 nm may bechosen.

[0028] In a preferred embodiment, Multimode Interference (MMI) couplerswith a 50:50 splitting ratio (commonly known as 3 dB couplers) make upthe couplers of the first stage 102 and 103, respectively, as well asthe couplers of the second stage 120 and 125, respectively.

[0029]FIGS. 2 and 2A show the second stage clock recovery circuit inisolation. The input 200 to this stage, at the top left of the figure,is the amplified RZ or PRZ signal output from the first stage. The stagecomprises a symmetric interferometer, with an SOA 210 and 215,respectively, in each arm. The interferometer has two DFB lasers astermini, DFB1 205, in lasing mode, and DFB2 220 near the lasingthreshold. This state of affairs results in an optical cavity that issensitive to the incoming input signal such that self-pulsating behaviorwill be triggered by any incoming data pulse.

[0030] The input signal 200, which has a large, usually far and awaydominant, frequency component at the original optical signal's clockrate, thus triggers the DFB lasers 205 and 220 into self pulsatingbehavior at that frequency, and the feedback between the two lasersresults in a pendulum like behavior that maintains the two lasers in aconservative self oscillatory state. This self oscillation is thusmaintained for some time, due to the mutual interaction of the lasers,even if the incoming data has numerous “zero” bits in a row (and thus nopulses at all for that interval). Thus the output signal of the secondstage 225 is an optical clock signal at the original line rate of theoptical input signal 100 in FIG. 1.

[0031] In general the clock signal can be “locked” on to after thesecond stage MZI has been fed ten (10) or more “one” bits from the inputsignal. As well, due to the conservative mutual feedback and selfoscillation of the lasers (which preserve their oscillation rate even inthe absence of continually added energy from the RZ/PRZ input signal110), the output clock signal 225 can be maintained even duringsignificant periods of no second stage input signal 200, such as in theevent of 100 “zero” bits, a statistically very rare occurrence, andunder some data formats, (where scrambling is done prior to transmissionover a link, and descrambling at the receiving end), quite impossible.Thus the mutual feedback and self oscillation of the two lasers presentsa robust structure for extracting a clean optical clock signal as itsoutput 225.

[0032] The method of the invention can be implemented using eitherdiscrete components, or in a preferred embodiment, as an integrateddevice in InP-based semiconductors. The latter embodiment will next bedescribed with reference to FIG. 3 FIG. 3 depicts a cross section of anexemplary integrated circuit SOA. With reference to FIG. 1, FIG. 3depicts a cross section of any of the depicted SOAs taken perpendicularto the direction of optical signal flow in the interferometer arms.Numerous devices of the type depicted in FIG. 3 can easily be integratedwith the interferometers of the preprocessor, so that the entire circuitcan be fabricated on one IC. The device consists of a buried sandwichstructure 350 with an active Strained Multiple Quantum Well region 311sandwiched between two waveguide layers 310 and 312 made of InGaAsP. Inan exemplary embodiment, the k of the InGaAsP in layers 310 and 312 is1.17 μm. The sandwich structure does not extend laterally along thewidth of the device, but rather is also surrounded on each side by theInP region 304 in which it is buried.

[0033] The active Strained MQW layer is used to insure a constant gainand phase characteristic for the SOA, independent of the polarization ofthe input signal polarization. The SMQW layer is made up of pairs ofInGaAsP and InGaAs layers, one disposed on top of the other such thatthere is strain between layer interfaces, as is known in the art. In apreferred embodiment, there are three such pairs, for a total of sixlayers. The active region/waveguide sandwich structure 350 is buried inan undoped InP layer 404, and is laterally disposed above an undoped InPlayer 303. This latter layer 303 is laterally disposed above an n-typeInP layer 302 which is grown on top of a substantially doped n-type InPsubstrate. The substrate layer 301 has, in a preferred embodiment, adoping of 4-6×10¹⁸/cm⁻³. The doping of the grown layer 302 is preciselycontrolled, and in a preferred embodiment is on the order of5×10¹⁸/cm⁻³. On top of the buried active region/waveguide sandwichstructure 350 and the undoped InP layer covering it 304 is a laterallydisposed p-type InP region 321. In a preferred embodiment this regionwill have a doping of 5×10¹⁷/cm⁻³. On top of the p-type InP region 321is a highly doped p+-type InGaAs layer. In a preferred embodiment thislatter region will have a doping of 1×10¹⁹/cm⁻³. The p-type layers 320and 321, respectively, have a width equal to that of the activeregion/waveguide sandwich structure, as shown in FIG. 3.

[0034] As described above, the optical signal path is perpendicular toand heading into the plane of FIG. 3.

[0035] Utilizing the SOA described above, the entire all-optical clockrecovery device can be integrated in one circuit. An exemplary method ofeffecting this integration is next described.

[0036] After an epiwafer is grown with the waveguide and the SOA activeregions, the wafer is patterned to delineate the SOAs, the AMZI and theMZI. In a preferred embodiment the path length difference between thetwo arms of the AMZI is approximately 1 mm.

[0037] Next, the DFB regions of the second stage of the device arecreated using either a holographic or a non-contact interferencelithographic technique. The periodicity of the grating in a preferredembodiment is approximately 2850A. The grating is of Order 1 andprovides optical feedback through second-order diffraction. The undopedInP top cladding layer, the p-type InP layers, and the contact layer arethen regrown on the patterned substrate. This step is then followed byphotolithography for top-contact metallization. The device is thencleaved and packaged.

[0038] While the above describes the preferred embodiments of theinvention, various modifications or additions will be apparent to thoseof skill in the art. Such modifications and additions are intended to becovered by the following claims.

What is claimed:
 1. A method of extracting the line rate clock from anoptical data signal of unknown bit rate or format, comprising:transforming the signal to cause it to have a high spectral component atits original clock frequency; and using the transformed signal totrigger optical oscillations at the original clock frequency.
 2. Themethod of claim 1, where the method is carried out completely in theoptical domain.
 3. The method of claim 2, where the method is carriedout with no loss of information;
 4. The method of claim 2, where theoptical oscillations are generated by using an interferometric devicewith two DFB lasers as termini.
 5. The method of claim 4, where theinterferometirc device is a MachZehnder interferometer.
 6. The method ofclaim 5, where the interferometric device utilizes a Multimode CouplerInterferometer.
 7. A method of retiming an optical data signalcomprising: transforming the signal to cause it to have a high spectralcomponent at its original clock frequency; generating an optical clocksignal from the transformed signal; and modulating the transformedsignal with the extracted clock.
 8. The method of claim 7, where themethod is carried out completely in the optical domain.
 9. The method ofclaim 8, where the method is carried out with no loss of information;10. The method of claim 8, where the optical oscillations are generatedby using an interferometric device with two DFB lasers as termini. 11.The method of claim 10, where the interferometirc device is aMachZehnder interferometer.
 12. The method of claim 11, where theinterferometric device is a Multimode Coupler Interferometer.
 13. Anoptical circuit, comprising: an interferometric device; opticalamplifiers in each arm of said interferometric device; and light sourcesterminating on opposite sides of said interferometric device.
 14. Theoptical circuit of claim 13, where said optical amplifiers aresemiconductor optical amplifiers (SOAs);
 15. The optical circuit ofclaim 14, where the light sources are distributed feedback (DFB) lasers.16. The optical circuit of claim 15, where a first DFB laser is lasing,and a second is close to its lasing threshold.
 17. The optical circuitof claim 16, where the first DFB laser is coupled with an input signal,and the second is coupled with the output signal.
 18. The optical signalof claim 13, where the interferometric device is a MachZehnderinterferometer.
 19. The optical circuit of claim 16, where theinterferometric device is a Mach-Zehnder interferometer.
 20. The opticalcircuit of claim 17, where the interferometric device is a Mach-Zehnderinterferometer.
 21. An optical circuit, comprising: a first stage,comprising: an interferometric device; optical amplifiers in each arm ofsaid interferometric device; and a delay element in one of the arms ofsaid interferometric device; and a second stage, comprising: any of theoptical circuits of claims 13-20.
 22. An optical circuit, comprising: afirst stage, comprising: a preprocessor, which converts an arbitraryoptical input signal to one in either RZ or PRZ format; and a secondstage, comprising: any of the optical circuits of claims 13-20.
 23. Amethod of restricting the RF spectrum of an optical data signal toeffectively that of the dominant frequency, comprising: inputting thesignal to an interferometric device; and using the signal to triggeroscillations between two light sources such that they keep each other inoscillation at the dominant frequency of the signal.
 24. The method ofclaim 23, where the two light sources each emit light at almost entirelyone wavelength.
 25. The method of claim 24, where the two light sourcesare distributed feedback (“DFB”) lasers, the first DFB laser in lasingmode, and the second DFB laser close to its lasing threshold.
 26. Themethod of claim 25, where the DFB lasers are each termini of theinterferometric device, the first DFB laser and the optical data signalbeing inputs to one side of the interferometric device, and the secondlaser being input to the other side of the interferometric device. 27.The method of claim 26, with an optical amplifier in each arm of theinterferometric device.
 28. The method of claim 27, where the opticalamplifier is a semiconductor optical amplifier.
 29. The method of claim28, where the semiconductor optical amplifier is the device of claim 32.30. The method of claim 23, where each arm of the interferometric devicehas an optical amplifier in the light path.
 31. The method of any ofclaims 23-30, where the entire apparatus is integrated on a substrate.32. A semiconductor device comprising: an InP substrate of a firstdoping type; a second InP layer of the first doping type disposed uponit; a third InP layer not doped disposed upon said second layer; a firstInGaAsP waveguide region laterally disposed on top of said third InPlayer, whose width is less than that of the substrate, first and secondInP layers; an active strained multiple quantum well (“SMQW”) regionlaterally disposed and centered on top of said first waveguide region,having the same width as said first waveguide region; a second InGaAsPwaveguide region laterally disposed on top of said SMQW layer, havingthe same width as said first waveguide region and as said SMQW region; afourth InP layer, undoped, disposed upon said second waveguide region,and extending downward, in the direction of the substrate, along thesides of said active region and said first waveguide region, whose widthis equal to that of the substrate, and the first and second InP layers;a first InP layer of a second doping type, laterally disposed above saidfourth InP layer, having the same width as said first waveguide regionand as said SMQW region; a seond InP layer of the second doping type,laterally disposed above said first InP layer of the second doping type,having the same width as said first InP layer of the second doping type;a contact layer laterally disposed above said second InP layer of thesecond doping type; and a metal electrode disposed above said contactlayer.
 33. An integrated optical circuit comprising: the optical circuitof claim 21; the semiconductor device of claim 32.